This invention relates to an improved method of recovering a salt from a multiple salt solution which is subjected to multiple effect evaporation. More particularly, this invention relates to an improved method of recovering a salt that has a solubility which increases with increasing temperatures from an aqueous solution containing another salt that has a solubility increasing less with increasing temperature by subjecting the solution to multiple effect evaporation having a plurality of evaporator effects operating at progressively higher temperatures.
A salt having a solubility which increases with increasing temperatures, hereinafter called a first salt, can be recovered from an aqueous solution containing another salt having a solubility increasing less with increasing temperature, hereinafter called a second salt. Water is removed from the solution, usually by evaporation, at such a temperature that the solution is brought to near its "invariant composition" without substantially precipitating the first salt. By "invariant composition" is meant the composition at which the aqueous solution is saturated with respect to two or more salts, e.g., saturation with respect to both the first salt and the second salt at a given temperature. The solution is then forwarded to a first salt recovery step, e.g., crystallization by cooling. In the process of removing water from the solution, second salt is usually precipitated therefrom if the second salt was initially in high enough concentration.
Potassium chloride, a first salt, is recovered from an aqueous solution containing both potassium chloride and sodium chloride, a second salt, by a process which entails first removing a portion of water from the solution by evaporation until the solution reaches its invariant composition. Evaporation is carried out at such a temperature that sodium chloride is precipitated preferentially owing to its solubility characteristic which causes it to be present in amounts exceeding its solubility while potassium chloride remains in solution. The sodium chloride depleted solution is then cooled causing the solubility of potassium chloride to be exceeded owing to its solubility characteristic thereby precipitating preferentially potassium chloride while the residual sodium chloride remain in solution. The invariant composition of potassium chloride-sodium chloride solutions is affected by other solutes in the solution. For example, a few parts of magnesium chloride per 100 parts of water will reduce the invariant composition of a potassium chloride-sodium chloride solution by a few parts of each salt per 100 parts water at a given temperature. Thus, evaporation is carried out with this factor in consideration.
Concentration of potassium chloride-sodium chloride solutions entails passing the solution through a plurality of evaporator effects operated at progressively higher temperatures in the direction of the flow of the solution concentrated therein, often described as backward feed. Each evaporator effect is in communication with an elutriation leg at the bottom thereof. Each evaporator effect is typically heated by steam forwarded from effect to effect opposite to the flow of solution being concentrated. The first evaporator effect is heated by steam from an external source, and the vapor therefrom heats the second evaporator effect. Vapor from the second evaporator effect heats the third evaporator effect and so on. Thus, the first evaporator effect operates at the highest temperature and the last evaporator effect operates at the lowest temperature.
Raw feed solution is fed into the last evaporator effect wherein it is concentrated with respect to potassium chloride and sodium chloride. If sodium chloride is in high proportion, as evaporation occurs the solution will first become saturated with respect to sodium chloride which will be the only salt to precipitate until the solution becomes saturated with respect to potassium chloride. Thereafter, and in subsequent hotter evaporator effects, the solution remains saturated with respect to sodium chloride notwithstanding higher solution temperatures because its solubility increases little with increasing temperatures. Hence, further evaporation at the higher temperatures will cause further precipitation of additional sodium chloride and further concentration in the solution of potassium chloride. Consequently, a large quantity of sodium chloride can be precipitated during evaporation. To avoid accumulation of this precipitated sodium chloride in the evaporators, slurry rich in precipitated sodium chloride is withdrawn from each evaporator effect. The withdrawn slurry is forwarded to a recovery step in which sodium chloride is removed or recycled to hotter evaporator effects. See, for example, U.S. Pat. No. 3,365,278 and U.S. Pat. No. 3,704,101.
In the cooler evaporator effects, e.g., third or fourth evaporator effect in a four effect evaporator system, the solution therein is near saturation with respect to potassium chloride. Due to raw feed salt composition fluctuations, various stream temperature fluctuations and other factors, operation at or near the invariant composition results in excess evaporation thereby causing precipitation of potassium chloride along with the precipitating sodium chloride. To avoid losing potassium chloride as a contaminant in the sodium chloride in the sodium chloride recovery step, the slurry withdrawn from the cooler evaporators (which is rich in precipitated sodium chloride but nevertheless contains an appreciable content of precipitated potassium chloride) is forwarded to the hotter evaporators where precipitated potassium chloride is redissolved. Thus, the slurry withdrawn from the hotter evaporator effects, e.g., the first and second evaporator effects in a four effect evaporator system, is forwarded to the recovery step where sodium chloride is removed. Consequently, potassium chloride is kept in solution in the hotter evaporator effects by maintaining the solution in the hotter evaporator effects at less than 100 percent saturation with respect to potassium chloride, i.e., the solution is maintained between 85 and 98 percent saturation with respect to potassium chloride. Thus, virtually no precipitated potassium chloride is forwarded to the recovery step where sodium chloride is removed since virtually no precipitated potassium chloride is withdrawn from the hotter evaporator effects.
More potassium chloride can be recovered in the recovery step where potassium chloride is removed, however, if the hottest evaporator effect were allowed to be maintained at 100 percent saturation or above and potassium chloride precipitated thereby is somehow reclaimed.